1. Field of the Invention
This invention pertains to surgical procedures for stabilizing the spine. More particularly, this invention pertains to a novel implant for use in such a procedure.
2. Description of the Prior Art
Chronic low back pain is one of the most common and perplexing problems facing the field of orthopedic surgery. In addition to patient discomfort, chronic low back pain has severe adverse societal impacts including lost income, possible chronic dependence on drugs, alcohol and public relief programs.
In many cases, low back pain can be avoided by preventing relative motion between spinal vertebrae (commonly referred to as intervertebral stabilization). To abate low back pain, stabilization is directed to stabilizing contiguous vertebrae in the lumbar region of the spine.
Surgical techniques are known for use in spinal stabilization. Surgical techniques seek to rigidly join vertebrae which are separated by a degenerated disk. Ideally, the surgery effectively replaces the vertebra-disk-vertebra combination with a single rigid vertebra. Various surgical techniques have developed which attempt to approach or approximate this ideal.
One technique known in the art is to partially remove a degenerated disk and to insert a bone graft into the void formed by the removed disk. Other techniques involve the use of an implant which, acting alone or in combination with bone fragments, replace the use of bone grafts. An example of such implant is shown in U.S. Pat. No. 4,501,269 to Bagby dated Feb. 26, 1985. In Bagby, a large, cylindrical basket is driven into a hole formed between bones which are to be joined. The basket is hollow and is filled with bone fragments which are produced during a boring step. Bone-to-bone fusion is achieved through and about the basket. In Bagby, the hole for the Basket is slightly smaller than the diameter of the basket. This structure results in the spreading of the opposing bone segments upon insertion of the basket. This results in taughtness, which provides initial stabilization. Eventual fusion of the opposing bone segments results from bone growth through the basket.
Prostheses such as that shown in U.S. Pat. No. 4,501,269 are promising. However, improved implant design is necessary to enhance patient safety and the probability of a satisfactory recovery.
According to a preferred embodiment of the present invention, an implant is disclosed for insertion into a body formed between opposing vertebrae of an animal""s spine. The implant includes a rigid body having a leading end and a trailing end spaced apart along a longitudinal axis of the body. The body has exposed threads which are disposed between the leading and trailing ends. The threads are selected to engage vertebra material and draw the body along the direction of the axis upon rotation of the body about the axis. The body defines a chamber which is exposed through the body by a plurality of radially extending openings. The chamber may be filled with bone fragments which can fuse with the vertebra bone material through the openings. In one embodiment, a reinforcing rib is provided within the body.
In an alternative embodiment of the invention disclosed herein, a generally oval-shaped implant is disclosed which is hammered into an elongated bore between two opposing vertebrae. The oval-shaped implant has enhanced surface area contact between the vertebrae and provides greater integrity against rotational motion between opposing vertebrae.